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Dec 07, 2019, 10:24 PM
GloBroz PowerLab
1QwkSport2.5r's Avatar
Originally Posted by 1967Brutus
Thanks Hansen, and yes, that was my initial thought as well... Then again, on the other end of the scale (larger engines running increasingly hotter) the opposite effect was less noticable, so much so that it was not even visible to me.

So the bottom end is, that I simply don't know for sure.

I tend to agree, but here's the thing: whether they actually, and under comparable conditions do run hotter or not, I cannot really determine. The differences are not significant enough for me to say yes or no.

What I CAN determine, is that when measuring at basically the same point of the engine, the 2-stroke generally is capable of handling higher cylinder head temperatures.
Regardless of the reason for the high temperatures, I have seen a small 2 stroke gasser run perfectly normal while the cylinderhead temperature was 180 degrees C. I have seen that on glow/methanol as well as gasoline/spark.
Not saying that that is a "normal operating temperature" because it very much isn't, just saying that it did not seem to affect the running behaviour much when it occurred. It would probably have damaged either engine if I would have continued the abuse, and I ould not have noticed if not for the telemetry.

Fourstrokes on gasoline and spark seem to start acting up at around 145 deg CHT, and I suspect the reason for that is that the temperature of the exhaust valve leads to pre-ignition. The symptoms are very much like an extremely lean running glow engine, while mixture in reality absolutely is not lean. It starts to sound different and rapidly loses power.
The same seems to happen with fourstrokes on glow/methanol, at above 155 deg C, but to be honest, I have way less data on glow/methanol fourstrokes than I have on gasoline/spark.

Those temperature extremes happened when experimenting with various set-ups in helicopters, experiments with watercooling, etc etc, and I do NOT claim that 2-strokes actually run hotter under normal circumstances.
The argument has been that 2-strokes run hotter than 4-strokes. The assumption on my part has been while burning glow fuel and using glow ignition. This premise in and of itself is invalid most simply because of combustion efficiency and maybe too but less-so, the heat transfer properties of the cylinder metallurgy and the cooling surface area of the crankcase/cylinder jacket itself. Higher cylinder pressure means more power (more HEAT). How efficiently the heat can be removed from the piston and combustion chamber to the cylinder and then crankcase is going to be the real wildcard. Some crankcases are better heatsinks than others. Steel transfers heat less efficiently than aluminum or brass. So there are distinct possibilities that some 4-strokes could run cooler than 2-strokes, but that will depend on other variables in play. I think it would overall be reasonably fair to say - given the number of variables that can affect the outcome, it would (IMO) be quite reasonable to say a 2-stroke will in many/most cases run cooler than a 4-stroke regardless of fuel used; but the results would probably be more poignant using alcohol based fuels. That just seems like common sense to an idiot such as myself.
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Dec 08, 2019, 01:13 AM
Diesel Danny
danny mz's Avatar
There are a lot of variables, if's and buts to consider.

Such as:

A four stroke engine only fires half as many times as a two stroke engine for a given rpm.

A two stroke crankcase and bearings are being constantly cooled by the fresh fuel that passes through them (unlike a four stroke engine).

At typical two stroke rpm the frictional heat would be expected to be more than in a four stroke engine (bearings and piston).
But then a four stroke has more moving parts (gears, cams, rockers and valves) so how much heat do they contribute?

A four stroke engine has greater combustion efficiency than a two stroke engine and better scavenging so the instantaneous cylinder temperature and pressure (during combustion) would be expected to be higher than a two stroke engine of the same capacity.

(On this last point I have noticed that a four stroke exhaust pipe discolours very quickly after the first run, but then it has a lot less mass than it's two stroke equivalent) so undecided on this

A temperature probe (thermocouple) bonded to a model engine exhaust close to the cylinder could be interesting.

In reality over many years I have found that the one variable that one has no control over is often the biggest problem - The Operator!

Typically inexperienced, the issues range from poorly cowled and/or over-propped engines to crap fuel/tank location and no idea how to set the HS and LS needles plus a lot of wishful thinking.

Just a few ideas * Danny M *
Dec 08, 2019, 07:21 AM
Registered User
earlwb's Avatar
Well as I see it, both 2 stroke glow and 4 stroke glow engines run at about the same temperatures. A 2 stroke engine can waste upwards of 40% to 50% of the air fuel mixture out of the exhaust port. A four stroke engine is more efficient as it doesn't waste as much fuel, but the power stroke is every other revolution instead of every revolution like a 2 stroke has. Thus the differences tend to balance out and both types of engines wind up operating at about the same temperatures. Now granted there are numerous variables and what ifs happening. A big factor is whether the engine is a little too lean or a little too rich being a large factor for temperatures. But if the engines are mounted on a test stand, adjusted well, the temperature results would be about the same for both.

You will have to use s thermocouple type of temperature sensor and embed the device into the engine in a good location to get the best temperature readings. Infrared units have too many variables that affect the readings. Reflectivity being the one big factor that really affects the measurements. Using the glow plug washer type of sensor is a decent compromise though, for both types of engines.
Latest blog entry: yes I still fly airplanes too
Dec 08, 2019, 12:20 PM
AMA 46133
SeismicCWave's Avatar
Thread OP
These speculations has to stop. All theories and opinions. Come on guys I started this thread because of the theories and opinions. I have heard enough of them. None of them mean anything to me. So I went out and did some tests. So are the test scientific? No. Are the tests the end of all tests? No. However the tests showed you all one thing. Talk is cheap. So far I hear talk and no one has picked up the process and did any other testing so far. Why? Because testing needs work. Testing needs some one to get off their chairs in front of the computer and do something.

The idea of which engine runs hotter than the other did not bother me. There are just so many variables and no one can nail down how to standardize the variables. Inherently 4 stroke engines and 2 stroke engines are very different.

The result of these tests did not bother me one bit but what really bothered mean is how this thread is degenerated back to a bunch of conjecture and speculations again. Stop talking and start doing. Speculations do not advance the argument/debate one bit. All the opinions do is to fuel the fire of speculations. Test some of your engines. Then I will listen to your speculations.

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